The development of mobile communication services imposes higher requirements on the transmission capability of air interfaces of future wireless communication systems. Thus, how to design systems with high peak transmission rate and high capacity has become a huge challenge.
In order to utilize limited radio frequency resources more effectively, the Time Division Multiple Access (TDMA) technique, the Frequency Division Multiple Access (FDMA) technique and the Code Division Multiple Access (CDMA) technique have been widely applied in mobile communication. Based thereon, two major mobile communication networks, namely GSM and CDMA, have been set up. These three existing techniques have been put into ample applications in terms of technique, and the spectrum utilization ratio has risen to the maximum.
However, as spectrum resources get increasing limited, future mobile communication systems might resort to a higher frequency band, such as the frequency band of 5 GHz. A higher frequency band means more serious channel fade than the 2 GHz frequency band of 3G systems, and a high transmission rate demands a higher signal/noise ratio and better channel transmission conditions. Therefore, the coverage of a base station of a future mobile communication system must be far less than that of a base station of current 2G and 3G systems. According to relevant study results, typically a future base station has a radius of several hundred meters or less. Therefore, whether or not conventional cellular network structures can be adopted and new architectures can be designed in future systems so as to meet future systems' demands becomes a problem that is worthy of careful consideration.
The MIMO technology proposed on this premise further improves the channel capacity by using multiple transmitting antennas and multiple receiving antennas. When there are ideal channel parameters (which may be obtained through channel estimation using training sequence or pilot sequence) on the receiving end, information capacity of the system with multiple transmitting antennas and multiple receiving antennas is in direct ratio to the smaller value between the number of transmitting antennas and the number of receiving antennas. In case of Rayleigh slow fading channel, the MIMO technology applies a model of multi-element antenna elements and spatial and time diversity at both ends of a wireless link, so that the system capacity is greatly enhanced and the wireless link reliability is improved at the same time.
As is well known, the MIMO technology is suitable for wireless transmission environment with rich scattering and relatively independent spatial correlation between antennas. If the channel environment has relatively strong spatial correlation, such as in conditions of line-of-sight propagation, the MIMO performance will decrease on a big scale. This is a major problem restricting MIMO applications.
As discussed previously, considering that the coverage radius of a base station will become smaller and smaller, a distributed structure can be introduced in order to improve the system coverage. FIG. 1 shows a schematic view of the existing distributed structure. As shown in FIG. 1, three antenna-RF processing units AP1, AP2 and AP3 serve as three access points located at three positions closer to mobile stations MT1, MT2 and MT3, respectively, and a baseband processing unit 10 serves as a central processing unit which is a centralized processing apparatus. The baseband processing unit 10 can be connected with the multiple antenna-RF processing units AP1, AP2 and AP3 in transmission forms such as radio over fiber (RoF) or copper wire, to perform data modulation and demodulation on these antennas.
Different from one-to-one form in a cellular system, antenna-RF processing units and a baseband processing unit is in multiple-to-one connection and control form in a distributed structure. In the distributed structure as shown in FIG. 1, each of the antenna-RF processing units is provided with four antennas, and each of the mobile stations is provided with two antennas. Through measurement of spatial correlation, two antennas can be selected from different antenna-RF processing units to form 2×2 MIMO transmission. For example, the respective first antennas selected from the antenna-RF processing units AP1 and AP3 form 2×2 MIMO transmission with the mobile station MT2, while other antennas are closed to avoid interference. Since the antenna-RF processing units AP1 and AP3 are widely spaced from each other, and correlation between the selected antennas is very weak, MIMO channels constituted by multiple pairs of distributed-placed remote antenna-RF processing units can well meet conditions for spatial independence.
In case of multiple mobile stations, the above distributed structure using MIMO channels mainly utilizes time division or frequency division to meet transmission for multiple mobile stations. That is to say, in different slots or frequencies, antennas to be used are selected in accordance with spatial correlation, and the system serves different mobile stations, and other antennas are closed to avoid interference. In other words, only one mobile station will be served in each slot or frequency. This severely restricts the system capacity.
With technological development, some technologies capable of effectively improving the system capacity have been proposed in recent years, such as beam forming and space division multiple access (SDMA). The beam-forming technology, which is applied to base stations of mobile communication, utilizes antenna arrays to gather signal energy into a very narrow beam and converges transmission power in the mobile station direction to obtain link gain, and thus improves the antenna propagation efficiency, wireless link reliability and frequency multiplexing rate. The SDMA technology can increase the output of downlink link cells by simultaneously scheduling packets for mobile stations that utilize different beam service, and thus makes it possible for mobile users who are merely different in spatial locations to multiplex one identical traditional physical channel. In consequence, in a system using the SDMA technology, multiple mobile stations can use the same wireless channel resources at the same time, such as slot, frequency and code, and the mobile stations are separated through spatial filtering. In this regard, Japan has already set about SDMA test and commercial use on PHYS systems.